VCSEL devices are laser diode devices used in a variety of applications to generate optical signals. For example, in optical communications networks, VCSEL devices are often used to generate optical information signals that are transmitted over optical fibers of the network. The most common configuration of a VCSEL is one that includes a conducting n-type substrate, an n-type distributed Brag reflector (DBR) disposed on the top surface of the substrate (the bottom DBR), an active region typically comprising multiple quantum well (QW) layers disposed on top of n-type DBR, a p-type DBR disposed on top of the QW layers (the top DBR), an ohmic n-contact, and an ohmic p-contact. The ohmic n- and p-contacts correspond to the terminals of the VCSEL.
When an electric potential is applied across the terminals, electrons from the n-type layers that are adjacent to the QW layers and holes from the p-type layers that are adjacent the QW layers are injected into the active region where they combine to produce photons. This combining of holes and electrons in the active region to produce photons is a phenomenon known as spontaneous emission. As the photons pass out of the active region, they are repeatedly reflected by the top and bottom DBRs back into the active region, which results in more recombination of electrons and holes in the active region. This is a phenomenon known as stimulated emission. The repeated reflection of photons by the DBRs back into the active region provides the “pumping” action that leads to lasing.
The speed at which a VCSEL can be driven, or modulated, ultimately is limited by the onset of relaxation oscillation inherent to the operation of the VCSEL. The relaxation oscillation is a manifestation of the energy exchanged between the total photon and carrier populations when the laser is disturbed from a steady state condition. This energy exchange results in a damped optical output power oscillation at the relaxation oscillation frequency. The relaxation oscillation frequency is a function of the square root of the laser bias current. In general, the relaxation oscillation frequency, fR, for a given bias current is relatable to the maximum modulation frequency bandwidth at which a laser diode can be driven, defined by the figure of merit f3dB, by the expression:f3dB˜1.55*fR.Thus, the 3-decibel (dB) modulation bandwidth of the laser diode is limited to a value of about 1.55 times the relaxation oscillation frequency.
In directly-modulated VCSELs, fR is limited to less than 30 Gigahertz (GHz) due to limitations on photon density resulting in large part from device reliability requirements and material differential gain saturation. In addition, because fR changes with the VCSEL bias current, it is difficult to use equalization schemes to extend the data rate for large-signal digital modulation schemes such as the non-return-to-zero (NRZ) and pulse amplitude modulation (PAM) 4 modulation schemes.
Indirectly-modulated VCSELs do not have the above limitations on fR, but they raise other problems. For example, vertically-integrated electro-absorption (EA) modulators are too thin to produce a useful extinction ratio (ER). Vertically-integrated electro-optical (EO) modulators tend to interfere with the operation of the underlying VCSEL, and therefore have very limited ranges of modulation.
A horizontally-integrated EA modulator solves the problems associated with the vertically-integrated EA and EO modulators, but it does not remove the inherent limitations of an EA modulator in terms of wavelength dependence of ER and the tradeoff between this characteristic and insertion loss, which makes EA modulators less suitable for use with large-signal modulation schemes involving multi-level signal coding.
It would be desirable to provide a VCSEL having a horizontally-integrated modulator that overcomes the aforementioned problem of the horizontally-integrated EA modulator and that has the advantages of the horizontally-integrated EA modulator in terms of overcoming the ER and modulation range problems associated with the vertically-integrated EA and EO modulators.